This invention relates to a lobe-type mixer tube for a bypass jet engine in which a bypass flow delivered by a fan is mixed with a hot gas stream issuing from the core engine, said lobe-type mixer tube having essentially circumferentially equally spaced gutter-shaped, radially moderately alternately inward and outward directed ducts to guide both the bypass flow and the hot gas stream, said ducts each having an end formed by an essentially plane extreme section and the respective extreme sections of some ducts when viewed in a longitudinal section extending in the direction of flow being inclined vis-a-vis the extreme sections of the other ducts, so that the various ducts when viewed in the direction of flow issue at various points offset one from the other. For the known state-of-the-art, reference is made, e.g., to GB 2 160 265 A.
The efficiency, i.e. the specific fuel consumption of bypass jet engines can be improved by mixing the hot gas stream from the core engine with the cold outer mass flow of the bypass stream before the mixed gases are expanded in their flow through a nozzle. This phenomenon is witnessed by thermodynamics theory and essentially relies on the divergence of the isobars in the enthalpy-entropy diagram. Another benefit achieved by internally mixing together the two streams of a bypass jet engine is in the accompanying reduction of jet noise, which is an important factor especially in the takeoff stage of the aircraft powered by this engine. Theoretical considerations regarding the boost in efficiency and the reduction in noise that can be achieved by mixing, however, show that the potential has not been fully exhausted.
To improve the mixing action, use is normally made of lobe-type mixer tubes to increase the contact surfaces between the bypass flow and the hot gas stream and produce helpful swirls in the mixing zone. To improve the three-dimensional mixing in the mixing zone of a lobe-type mixer to be, scarfing is used, which denotes a configurational modification to lobe-type mixer tubes through which the radially inward directed ducts of the lobe-type mixer tube are alternately trimmed axially in a circumferential direction, or are left untrimmed or are even extended. A lobe-type mixer tube of this description is disclosed in the initially cited patent GB 2 160 265 A, where the respective ends of the ducts are essentially formed by plane, extreme sections and these extreme sections of some of the ducts are inclined vis-a-vis those of the other ducts.
The design of a lobe-type mixer tube generally constitutes a tradeoff between the requirement for maximum mixing efficiency and that for minimum pressure losses associated with it. The degree of mixing efficiency is affected by a number of constraints, such as the penetration of the lobe-type mixer tube referred to the diameter of the exhaust pipe surrounding it and the path length to the point where expansion occurs in the nozzle, referred to nozzle diameter. The resultant pressure losses are a combination of friction and deflection losses.
A broad aspect of this invention is to provide means on a lobe-type mixer tube in accordance with the generic part of claim 1 that enable the mixing efficiency to be enhanced without producing accompanying pressure losses.
Its is a particular object of this invention to provide an arrangement in which the radially moderately outward directed ducts, when viewed circumferentially, alternately issue at points offset from each other when viewed in the direction of flow. Further objects and advantages of this arrangement will become apparent from the subclaims.
In a further aspect of the present invention, four each circumferentially successive ducts form a group that repeats over the entire circumference and consists of a pair of radially moderately outward directed ducts and a pair of radially moderately inward directed ducts, one each duct of a pair being shorter and the other duct of the pair being longer than the other duct, where when viewed circumferentially, each longer inward directed duct is followed by a longer outward directed duct, which is followed by a shorter inward directed duct which in turn is followed by a shorter outward directed duct. Further objects and advantages will become apparent from the subclaims.
In a further aspect of the present invention, every other radially moderately inward directed duct is trimmed axially, or scarfed as described in the previously repeatedly cited patent specification GB 2 160 265 A. Additionally, however, every other radially moderately outward directed duct is scarfed as well. Accordingly, the extreme sections of a pair of circumferentially successive, radially inward directed ducts are inclined vis-a-vis each other, as are the extreme sections of a pair of circumferentially successive, radially outward directed ducts. In other words, both the radially moderately outward directed ducts and the radially moderately inward directed ducts come in two different lengths of duct, when taking the length of duct to be the length of duct wall or lobe-type mixer tube as measured in the direction of flow.
In accordance with the present invention, therefore, we practically have a group of four different ducts, i.e., one each shorter and one each longer moderately radially outward directed duct and one each shorter and one each longer moderately radially inward directed duct. In this arrangement, the ducts are disposed as indicated, i.e. in a circumferential view, a longer inward directed duct is followed by a longer outward directed duct, which in turn is followed by a shorter inward direct duct that is followed by a shorter outward directed duct. A lobe-type mixer tube in accordance with the present invention has several such groups circumferentially spaced around its circumference. Here, a periodic arrangement may be selected, where a first group is followed by a second group in which the ducts are arranged in the same sequence as in the first group. An axially symmetrical iteration may alternatively be selected, however, such that a first group is followed by a second group in which the ducts are arranged axially symmetrical with the first group.
These aspects of the present invention appreciably enhance the three-dimensional mixing effect and hence the mixing efficiency of the of lobe-type mixer tube. Considering that the surface area of a lobe-type mixer tube increases overproportionately when viewed radially outward, scarfing the lobe-type mixer tube as described on its outer side, i.e. at its radially moderately outward directed ducts, largely reduces the surface area, advantageously reducing weight and optimizing the pressure loss. The improved geometric flexibility improves the spatial distribution of the swirls induced at or in the lobe-type mixer tube, so that a more complex and hence optimal mixing process results.
It should here be remembered that the intended improvement of the mixing action is achieved also by the differences in length of said various ducts, so that to achieve the desired effect it may not necessarily be required to heavily trim, e.g., every other duct axially. Rather, some of the ducts may alternatively also be extended in length, while adjacent ducts may exhibit an extreme section that essentially extends in a direction essentially normal to the direction of flow. Whereas a weight advantage is optimally achieved by said axial scarfing. Also, the total number of lobes of the inventive lobe-type mixer tube should be a multiple of 4, allowing an accurate periodicity to be set, where a certain number of duct groups exists in the lobe-type mixer tube and each group is complete in itself, consisting of said four ducts of various lengths.